I had on period that was 3 weeks late. Then my next two were a couple days early. In between my last period and this one (around the middle of my cycle) I had two different days, about a week apart from each other that I had bad cramping and back pain. No bleeding. Then my period came 3 days late. I have never been regular, but this pain in between is not normal for me. What could be causing this? I’m not pregnant.